What is alt-text, and what does it do?

Alt-text is a piece of text that takes the place of an image if the image itself cannot be seen. This can be due to a disability or due to technical limitations.

Blind or visually-impaired people use screen reader software that reads text out loud for them. If there is an image with an alt-text, the screen reader reads out the alt-text where the image is. If the image has no alt-text, the screen reader simply says, "Image" or "Graphic" or something like that. It can't automatically describe the image, nor can it read out text in an image.

If someone cannot see an image due to technical limitations, and the image has an alt-text, the alt-text is shown in the place of the image. But if the image has no alt-text, nothing is shwon in its place.

In general, alt-text should be short, not significantly longer than 200 characters although there is no technical limit anywhere in the Fediverse that is that short. Long alt-texts are inconvenient for screen reader users. See, when a screen reader reads any plainly visible text such as a post text out loud, it can navigate back to any point in the text, e.g. a few words back. But it can't do that when reading out an alt-text; it can only jump back to the beginning of the alt-text and then re-read the whole alt-text.

Alt-text has only got one function, and that is to describe what the image shows, usually as far as it's relevant within the context in which the image is posted.

Alt-text must not contain explanations or contact information or license information or Easter eggs or anything else that is neither available in the post text nor in the image itself. Likewise, alt-text must not be used by Mastodon users to write around their 500-character limit.

This is because not everyone can access alt-text, just like not everyone can see the images in your posts. Those who can't access alt-text can't access anything that you only offer in your alt-text at all.